The invention relates to methods and apparatus for medical examination by means of ultrasound echos. More specifically, the invention relates to methods and apparatus for compensating the characteristics of ultrasound receivers to eliminate distortion which may be introduced by variations in attenuating characteristics of body structures.
Ultrasound pulse echo techniques have been utilized in the prior art to locate, image, determine the motion of, and otherwise characterize internal structures of the human body. In such systems, a pulse of ultrasonic energy is caused to propagate into the body, where it may be reflected by discontinuities along its propagation path (such discontinuities may, for example, occur at the boundaries of body organs) The time and magnitude of the reflected echos are measured and utilized, for example, to produce cross-section images (B-scans) of internal body structures. The techniques utilized in medical ultrasound systems are similar to those used in sonar systems, however special problems result from the significantly smaller size of the structures to be examined and from the unusual attenuating characteristics of body tissues.
It is known, from the prior art, that body tissues on the average attenuate ultrasound energy at a rate of approximately 1 dB./mHz/cm. To compensate for this rapid attenuation, prior art ultrasound systems generally incorporate circuitry (termed "time-gain compensation" or TGC circuitry) which acts to increase receiver gain during a time period which follows the introduction of a pulse into the body. The time required for reflected pulses to return to the receiver is a direct function of the depth of a reflecting structure within the body; thus receiver gain is automatically increased for reflected pulses which originate deep within the body. The attenuation of ultrasound energy is, however, not uniform for all body tissues and structures. The body wall is substantially more attenuating than average body tissues while fluid filled structures, for example cysts, are substantially less attenuating than average body tissues. The term "cyst", as used herein and in the claims which follow, refers to any fluid filled cavity within the body and includes, for example, the urinary bladder and, in some cases, larger blood vessels.